1. Field of the Invention
The present invention relates to a video camera apparatus and, more particularly, to an arrangement suitable for use as a lens controlling device for a video camera.
2. Description of the Related Art
The recent development of video instruments such as video cameras, electronic still cameras and camera-integrated VTRs is remarkable. In particular, the functions and operability of such video instruments have been greatly improved and their size and weight have been increasingly reduced. Among others, camera-integrated VTRs have been rapidly gaining in popularity, and great reductions in their size and weight have been realized owing to the minimization of the number of parts used per VTR as well as changes in the structures of the VTRs themselves.
As a viewfinder used for monitoring an image being photographed, an electronic viewfinder utilizing a small CRT, a liquid-crystal display or the like has been adopted instead of a conventional optical viewfinder. Such an electronic viewfinder not only permits monitoring of an image which is being photographed, but is useful for improving the performance of various operations such as the reproduction of recorded image information or the displaying of various kinds of information.
For example, a typical camera-integrated VTR includes a lens unit which requires relatively large space and parts. To minimize the size of the lens unit, a structure called an inner focus type, such as that shown in FIG. 1, is suitably utilized.
In this type of lens unit, a front lens element is fixed in position and rear lens elements are used to vary magnification or to adjust focus so as to minimize the size of the lens unit.
The lens unit shown in FIG. 1 includes a fixed front lens 101, a magnification varying lens (zooming lens) 102, a n iris 103, a fixed third lens group 104, and a fourth lens group (focusing lens) 105 which performs a focusing function and th e f unction (compensator function) of compensating for the movement of a focal plane due to the movement of the zooming lens 102. The operational characteristics of the lens unit will be described below.
As magnification is varied by moving the zooming lens 102 in the lens unit arranged as shown in FIG. 1, the fourth lens group 105 operates to perform the compensator function and the focusing function as described above. The manner of this operation is shown in FIG. 2.
FIG. 2 shows the positional relation between the zooming lens and the focusing lens with a subject distance as a parameter, and the horizontal axis represents the position of the zooming lens, while the vertical axis represents the position of the focusing lens. As is apparent from FIG. 2, during zooming, if the focusing lens moves along a locus unique to each subject distance, it is possible to continue the zooming without defocus, i.e., in an in-focus state. If the movement of the focusing lens deviates from the unique locus, defocus will occur.
A method of moving the focusing lens along a locus unique to each subject distance during zooming is proposed in, for example, Japanese Laid-open Patent Application No. Hei 1-280709. In this method, the loci shown in FIG. 2 are divided into zones each including a group of loci drawn at an approximately equal inclination, as shown in FIG. 3, and one speed is assigned to each of the zones as a representative speed. During zooming, any one of the zones is selected on the basis of the positional relation between the zooming lens and the focusing lens, and while both lenses are positioned within the selected zone, the focusing lens is made to move at the representative speed assigned to the zone.
However, the above-described method has the problem that the representative speed for each of the zones is determined with respect to a single zooming-lens moving speed and if the zooming-lens moving speed varies due to, for example, a variation in a zooming-motor output, a temperature change, a change in the attitude of the lens unit due to a change in a camera angle or the like, the focusing lens does not correctly follow the loci of FIG. 2.
Japanese Laid-open Patent Application No. Hei 1-319717 proposes a method of adjusting a focusing-lens driving speed during zooming by increasing or decreasing a coefficient to be multiplied by the aforesaid representative speed in accordance with a change in an actual zooming speed.
Referring to FIG. 3, for example, the horizontal axis is divided into 16 equal parts. If it is assumed that the design driving speed of the zooming lens is set to a speed which permits the zooming lens to move between a telephoto end (T) and a wide-angle end (W) in 7 seconds, 26 vertical sync periods (26 Vsync) are required for the zooming lens to pass through a single zooming zone 801 as shown in FIG. 4 in the case of the NTSC system.
In general, if N [Vsync] is taken to pass through the single zone during actual zooming, the change ratio Rzs of the actual zooming speed to a reference value (T.revreaction.W: 7 sec) of the zooming speed is expressed as: EQU Rzs=N/26 (1)
Accordingly, during zooming, by always measuring the number of vertical sync periods required to pass through the aforesaid single zone and multiplying 1/Rzs by the aforesaid representative speed, it is possible to perform the zooming at a focusing-lens moving speed according to a variation of the zooming speed without defocus.
In a case where zooming is initially performed after the power source of the apparatus is turned on, there is no measurement data on the zooming speed and no correct value obtained from an actual zooming-lens driving speed is inputted as Rzs.
The above-described kind of apparatus has additional problems because the measurement of the zooming speed or calculations on Equation (1) have been performed by a microcomputer. If measured values or measurement results are stored in a volatile memory, the stored data are lost when the power source is turned off, and are not used for later control. If zooming is initially performed with data remaining lost after the power source has been turned on, there is no measurement data on the zooming speed and no correct value obtained from an actual zooming-lens driving speed is inputted as Rzs. As a result, focusing control does not respond to the zooming until a stable measured value N is obtained, and the zooming may start at an utterly different focusing-lens speed.
To compensate for the disadvantage, data may be stored in a non-volatile memory such as an EEPROM. However, if the lens unit is not in use for a long time or if an environment or the aforesaid attitude changes when the power source is again turned on, the zooming speed may change, causing zooming to start at an erroneous focusing-lens driving speed.
To solve the above-described problems, it is necessary to perform various control after the relation between each driven part of a lens optical system and its detecting system is reset to its initial state before photography is started or when the power source is turned on.
To meet the necessity, there is proposed a method of performing a lens resetting operation, i.e., causing the zooming lens to move by the minimum amount required for measurement immediately after the power source has been turned on, determining the initial value of Rzs, finding an actual focusing-lens speed by multiplying this value of Rzs by a representative speed corresponding to the zone where the zooming lens is located, then returning the zooming lens to its original position, and subsequently establishing ordinary operating conditions.
The aforesaid method will be explained below in more detail. It is assumed here that a stepping motor or a similar means which is not easily influenced by inertia and which provides a constant amount of drive with respect to a drive signal and has a wide speed response range, is used as an actuator which constitutes a focusing-lens driving means. In this case, to detect the position of the focusing lens, a method is available which includes the steps of counting the number of drive pulses outputted from the stepping motor by using a counter and causing the count to correspond to the position of the focusing lens. In this method, the counter serving as a position encoder is an incremental counter, and to cause the count to correctly correspond to the coordinate of the vertical axis of the locus diagram of FIG. 2, from the time the power source is turned on until the time an ordinary photographic operation is started, it is necessary to execute control to move the focusing lens 105 to a predetermined position, substitute a value corresponding to the predetermined position into the counter with the focusing lens 105 located at that position, and start counting for detecting the position of the focusing lens 105 which varies with the movement thereof.
In other words, it is necessary to perform various control after the relation between each driven part of the lens optical system and its position detecting system is reset to its initial state before photography is started or when the power source is turned on.
However, in the aforesaid system, while the reset operation of the lens optical system is being performed, the zooming lens moves to cause a variation in a field angle which does not conform to the intention of an operator, whereas the focusing lens moves beyond an effective infinity end to cause a great extent of defocus. As a result, the quality of an image remains seriously degraded until the reset operation is completed.